i) Field of the Invention
The invention relates to a process for recovering silver metal from an ionic silver-bearing spent photographic solution; and in an especially preferred embodiment exploits reduction of silver ions to silver metal with a photographic developer reagent as the reducing agent.
ii) Description of Prior Art
The commercial processing of photographic materials, industrial X-rays, medical X-rays and graphical arts produces spent solutions containing silver ions. Environmental regulations restrict the discharge of such solutions to within acceptable concentrations of silver that are much less than the concentrations of silver in the spent solutions. Various methods of removing silver from these solutions have been attempted, with varying expense and success.
The most common types of silver recovery in current use include the use of steel wool in a chemical replacement cartridge (CRC), electrolysis, ion exchange, and a precipitation method. Some CRCs are messy and the effluent therefrom can contain high concentrations of iron. Electrolysis is expensive, requires significant maintenance, and can result in ammonia gas generation. Ion exchange is costly and impractical for smaller photo processors and minilabs. Employment of sodium sulfide as a precipitating agent can result in the evolution of toxic fumes. Another approach has been use of a silver complexing agent to remove silver from photographic effluent solutions, however, complexing agents are often costly.
A problem with these approaches is that separate separation facilities and equipment are required, which can be inconvenient and expensive, particularly for small photoprocessing labs or minilabs. Furthermore, these processes do not provide a process capable of recovering silver from minilab effluent solutions such as photographic stabilizers to obtain a sufficiently low silver concentration acceptable for discharge to the environment.
U.S. Pat. Nos. 4,662,613 and 4,325,732 of Woog and U.S. Pat. No. 5,900,041 of Riviere and Ren describe recovery of silver by ion exchange between silver ions and iron metal. These processes result in an iron ion bearing discharge effluent from which the iron ions often precipitate as iron oxides.
U.S. Pat. No. 5,288,728 describes the use of a trimercapto-s-triazine complexing agent to precipitate silver in a silver complex. While the method lowers the silver concentration in the effluent, the cost of the complexing agent is significant, additionally it is a man made chemical with unknown long term effects on the environment.
No prior art is known that employs an organic reducing agent as the primary means of recovering silver from an ionic silver bearing spent photographic effluent.